Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

M-Theory is a theory which 'combines' the five superstring theories

  1. Jun 1, 2005 #1
    Waddup guys. To my understanding M-Theory is a theory which 'combines' the five superstring theories and supergravity.Also i understand that matematics is not yet advanced enough to make it work(or so ive been told) Im still not sure how this theory is meant to end up as a theory of everything. You thoughts?
  2. jcsd
  3. Jun 1, 2005 #2
    When 'theory of everything' is banded about, it means a theory which unites all the four fundamental forces into one common framework. The forces are, of course, electromagnetic, strong, weak and gravitational.
  4. Jun 1, 2005 #3
    I believe a TOE would also explain where spacetime itself came from and how particles emerged from that.
  5. Jun 2, 2005 #4
    Yea thanks i already know what a TOE is and the four fundamental forces are thanks ne way
  6. Jun 2, 2005 #5
    Gravity is thought to be curvature of spacetime itself. But String Theory and M-theory are various dimensional submanifolds embedded in a background spacetime. Gravitons are thought to be some type of string or brane. So there seems to be a contradiction. Strings and branes are thought to be gravitons of gravity which is thought to be spacetime itself in other theories. So where did the background come from in the first place?
  7. Jun 16, 2005 #6
    So what if they find a background independent formulation of string/M-theory? What would that mean? You would still have gravitons and all other particles explained in terms of this background, i.e. vibrations of something within a background space, right? The formulation would simply be background invariant, not depend on whatever spacetime is chosen in which to describe the theory, right? But that would still REQUIER a background, which the theory would not explain. Is that correct?
  8. Jun 16, 2005 #7


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    All the difference in the world between particles flying around in a curved, fixed, background space on the one hand, and particles causing the spacetime to bend, while it causes them to curve in their paths. Background independent means that the space is in the foreground, taking part in the physics, and dynamically altering and being altered.
  9. Jun 16, 2005 #8
    How can gravity be both a curved spacetime and a particle moving through spacetime simultaneously? If gravitons are particles moving through a background spacetime, then where did the background come from? Even if it doesn't matter what the background is (it still results in the same physics), you still seem to need a background in order to calculate the particle properties. Where did that background come from? Is there a 5th force which does bend the background spacetime? If it doesn't matter what the background is in order to obtain the particle properties, then the particle properties cannot depend on the background, so the background, likewise, cannot be influenced by the particles, right?
  10. Jun 16, 2005 #9


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    All good questions. Nobody knows. If we quantize gravity by any method, there will be quanta of gravity. If our quantization is background independent - that is, it is a quantization of spacetime itself - then these quanta of gravity will also be quanta of spacetime. Einstein thought of "the field" as replacing spacetime, and there have been others who thought of spacetime as replacing the field! Quantize that!
  11. Jun 16, 2005 #10
    You understand the contradiction, don't you? You assume a continuous spacetime in order to calculate the quanta of spacetime. So which is it, right?

    I'm trying to understand the meaning of background independence. Doesn't that mean an invariance of some sort, that the physics is the same no matter what the curvature of spacetime is. Is this right? I understand the term independence to be invariance, but you suggest it means quantization. Are you saying that invariance equal quantization. Maybe there is a principle to this effect that I forgot.
  12. Jun 17, 2005 #11
    If you start with nothing, and then make two somethings out of it, it should not surprise you if, when you add the two somethings together again, you get nothing. Does this mean that neither of the two somethings exists on its own? Of course not. This is the value of separation. A positron and an electron can be perfectly good somethings, until they happen together. Then they return to nothing again. The energy, or action potential, which results and propagates, wavelike, is not something, in itself, but only the reactions of the other somethings. Not?

    Be well,

  13. Jun 17, 2005 #12
    Yes, I've often thought about this. I think it is expressed as a conservation of information law in the universe, though I don't have any proof of this. What I mean is that whatever structure exists is describable by some mathematical means. Its construction stands opposed to nothingness, obscurity, and chaos. There is information associated with its being, information necessary to describe it. However, there is also dissipative effects going on in the universe. For example, it might be that the expansion of the universe itself is a dissipative effect that increases the entropy in the universe. With expansion, more possible states exist for things to dissipate into. So there are dissipative effects that increase entropy which causes information loss, and there are constructive events which decrease entropy which increases information. It may be necessary for new, more complex structures to arise as the universe expands.
  14. Jun 20, 2005 #13
    String/M - theory does not explain where the background spacetime comes from to begin with, nor does it explain how the string/particles arise from the background. So of course their having problems with how spacetime is defined, which Calabi-Yau manifold is correct. And of course their having trouble deriving the mass of the particle. These problem seem inherent in how they approach the problem- that strings vibrate in the background, without defining the background or how mass emerges from it.

    This inherent problematic approach may also affect Causal Dynamical Triangulation. For CDT also does not define where the background came from to begin with, nor does it say how mass arises from that background. So they too will probably not determine the correct ground state nor the masses of the particles.
  15. Jun 25, 2005 #14
    The M-theory does not state that it is the "T.O.E." or Unified Theory. Although it may be used to determine various possibilities and theories, but it is NOT a Unified Theory. There are countless properties of nature it cannot explain.
    For more info on this subject, I recommend http://en.wikipedia.org/wiki/M-theory
  16. Jun 26, 2005 #15
    The first sentence of your reference reads: M-theory is a solution proposed for the unknown theory of everything which.......

    Anyway, string theory isn't even able to calculate the thing it was originally investigated for, the masses of the particles. I wonder if this is not something problematic in the approach. Without first explaining how strings emerges from the underlying bulk, it may be that we are given a problem without initial conditions to calculate a unique solution. And this allows many different solutions from which we have no method of choosing the correct one. So we have the problem of the "Landscape".

    And similarly, we cannot describe how strings or branes emerge from the background spacetime until we know what the background spacetime geometry is. So in order to get the masses, we need to know the background geometry which we cannot solve for until we have the initial conditions for that.

    It seems at this point we may have no other option than to try to start from the very beginning of spacetime itself in order to make any progress at all.

    Or is it the case that we are trying to find the initial conditions from the final solution, given the mass of particles can we find out how mass came into being in the first place. Is there an analogous situation from differential equations where we can derive the t=0 conditions from the t=something conditions? It seems problematic though to try to derive the given conditions from the given conditions. There is no way of knowing that your diff eq is correct to begin with, right?
  17. Jun 26, 2005 #16


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Whether or not M-theory will ever become a Theory of Everything, a theory that really did what the M-theorists would like to do - unify all four of the basic forces - would deserve the name. What are the "countless properties of nature it cannot explain"? emergent ones? They are still causally underpinned by particle physics. See self-organized criticality.
  18. Jul 1, 2005 #17
    Would String/M-theory explain where spacetime came from to begin with? Would it explain how particles emerged from the background to begin with?
  19. Jul 1, 2005 #18


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Maybe it would. Consider the Ekpyric hypothesis as an example showing it could. And I believe present day physics has something to say about how the particles emerged.
  20. Jul 2, 2005 #19
    Hi Mike2

    You ask "Would String/M-theory explain where spacetime came from to begin with?" This is not an uncommon question. It gets asked a lot in one form or another. To begin, let me make the assumption that String/M-theory certainly would answer the question if it could. Could String/M-Theory answer the question? I believe I know the answer to that, even though I am not able to function as a String/M-theorist. No.

    Let me rephrase the question just a little, to generalize it to a more fundamental question. Where did spacetime come from to begin with? You see I have removed the part about String/M-Theory. I don't mean to avoid the intent of your quetion, as I hope to show in a moment.

    Where did spacetime come from to begin with? I am sure you don't mean, "Who thought of the idea of spacetime?" I think you are asking about the physical origins of the physical observables.

    Then consider the set that contains all of space and all of time and all of the relationships between space and time. This set contains the first time and all of the space present at the first time, as well as the last time and all of the space present at the last time, and all of the times and all of the spaces between the first time and the last time. Is there any space or any time or any spacetime that is not contained in the set of all spacetime? I think you will agree that there is not and in fact cannot be. Now let us return to the form of your question. You ask where spacetime came from to begin with. Where? Surely the place must be contained in the set of all spacetimes. And when, as you say, to begin with? Surely that time must also be contained in the set of all spacetime.

    So the question, it seems to me, has no meaning as it is stated. You are asking, what part of spacetime is not part of spacetime!

    Surely String/M-theory cannot answer a question that has no meaning. So the answer to your original question as originally stated can only be, No.

    I don't want to seem harsh. You have shown great merit in attempting to think about these difficult things. But if you really want an answer, you have to go deeper. You have to think about what spacetime is. You have to transform your question, and your vision. You have to learn to think as a creature who inhabits more than our common three dimensions of space and one of time.

    And there is so much more.

    I wish you well. Have fun thinking.

    Last edited by a moderator: Jul 2, 2005
  21. Jul 2, 2005 #20
    Hi selfAdjoint

    I don't find anything on Google about Ekpyric hypothesis. It asks if I mean Ekpyrotic hypothesis, which seems to have something to do with theology. I don't have much interest in studying things that cannot be known so I did not go there.

    Ekpyric. Out of the fire, as my memory of Greek serves me? Or maybe, just out of fire. That sounds right, since I find in the literature (Weinberg, The First Three Minutes, for example) that particles are thought to have condensed out of the cooling, expanding energy ('fire') sometime after the big bang.

    Could you be more specific? I miss chatting with you here.

  22. Jul 2, 2005 #21
    nightcleaner, are you just saying that the question is meaningless simply because outside of the "set of all spacetime," space and time isn't defined? Which would be like asking what happened before the beginning? It's a meaningless question because if time was created at the beginning, there conceivably can't be a "before the beginning" if there's no conception or notion of time 'before' it was created.

    (Spacetime, in The Elegant Universe, is defined as "A unsion of space and time originally emerging from special relativity. Can be viewed as the 'fabric' out of which the universe is fashioned; it constitutes the dynamical arena within which the events of the universe take place.")

    Aside from this matter, M-Theory does explain where spacetime "came from," or it's origins. M-Theory makes usage of cosmic membranes that forms a multiverse, or a collection of many universes.

    Michio Kaku explains in Parallel Worlds that the universal law that everything abides by is the quantum principle. He says that even when there is nothing at all, the quantum principle remains, and according to the quantum principle, even nothing is unstable. Before there was anything, there was nothing. This 'nothingness' bubbled with uncertainty due to the quantum laws and these bubbles spontaneously and arbitrarily expanded into a great multitude of baby universes. Michio Kaku conjectures that from this sea of bubble universes, our universe was formed, but of which is only one out of the myriad of universes formed in the beginning from the quantum laws.
    Last edited: Jul 2, 2005
  23. Jul 2, 2005 #22
    Hi Sempiternity

    Well, yes, that is what I am saying, but beyond that, I believe what is required for better understanding of the origins of measurable phenomena, is a view which transcends our customary view of three dimensions of space and one of time. "Spacetime" is a concept which was advanced by Einstein and Minkowski to indicate the equality of space and time, in which questions about what came before the beginning or what lies outside the boundaries of our universe become meaningless. Better questions are required to explore the idea of what, if anything, lies outside the definition of spacetime.

    I would like to try to encourage people who ask this kind of question to evolve beyond the 3 space 1 time mindset. I am sure you will agree that the branes and the bulk in which they are said to reside are not adequately described by three dimensions of space and one of time. Quantum theory requires us to study imaginary numbers, the complex plane, non-commutative algebras, higher dimensional geometries, and other mathematics which go beyond our ordinary visualizations of reality.


  24. Jul 2, 2005 #23
    Thinking "outside of the box" is certainly something that must be done when dealing with such unusual ideas and concepts. It's a mind-bending journey attempting to grasp the conceptuality of multiple dimensions and fundamental origins, which makes your encouragement very well-founded :)
  25. Jul 2, 2005 #24
    Of course I meant "How did spacetime come into existence to begin with" and not where or when. The only answer is that it grew from a singularity. How it grew, by classical or quantum means I don't know.

    Branes colliding to form our present spacetime only beg the question as to where the branes come from and how the spacetime in which the branes move came into existence.

    I can't accept that spacetime arose from nothing through quantum effects. For quantum effects only effect things that exist, or at least can be calculated as if they exist. It seems quantum mechanics can only be done by using a background of space and time. Thus it assumes the existence of spacetime and so cannot be used to explain how spacetime emerged to begin with.
  26. Jul 2, 2005 #25
    On page 94 of Parallel Worlds, Michio Kaku further explains the concept of something from nothing:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook